1. Field of the Invention
The present invention relates to a fully hydraulic power steering apparatus for industrial vehicles such as lift trucks, and more particularly, relates to a fully hydraulic power steering apparatus provided with a means for correcting an error between an angle of turn of a steering wheel and a pivoting angle of tires of rear wheels on an industrial vehicle. Note, it is to be understood hereinafter that the expression "fully hydraulic" means that a hydraulically operated power steering apparatus does not include a mechanical connection between a steering wheel and wheels to be pivoted for steering an industrial vehicle.
2. Description of the Related Art
In the field of industrial vehicles, a fully hydraulic power steering apparatus is now employed to enhance the manual operability of a steering system of the vehicle. FIG. 12 illustrates a typical example of such a conventional fully hydraulic power steering apparatus for an industrial lift truck. The power steering apparatus of FIG. 12 is provided with a power steering valve unit 52 operated by a steering wheel 51, a double-rod type hydraulic power cylinder 54 for causing a pivoting of rear wheels 55 (in such industrial vehicles, the rear wheels are pivoted by the steering gear) to thereby steer a vehicle having the steering wheels 55, and hydraulic lines 56 and 57 interconnecting the power steering valve unit 52 and the power cylinder 54. When the steering wheel 51 is turned by a driver, the power steering valve unit 52 admits a part of pressurized oil delivered from a hydraulic pump 53 into the power cylinder 54, and accordingly, the power cylinder 54 causes a pivoting of the rear wheels 55. The amount of the part of the pressurized oil admitted is proportional to an angle of turn of the steering wheel 51. Nevertheless, since the above-mentioned fully hydraulic power steering apparatus does not provide a mechanical relationship between the steering wheel 51 and the rear wheels 55 to be pivoted, often an error appears between the angle of turn of the steering wheel and the pivoting angle of the rear wheels corresponding to an amount of stroke of the rods of the power cylinder, due to a leakage of the pressurized oil within the power steering valve unit 52 caused by repeated steering operations. Accordingly, in a forklift truck provided with a steering wheel 51 having a knob 51a, the above-mentioned angular error discomforts the driver because the neutral position of the steering wheel 51 is gradually moved from a predetermined position at which it should be located during a straight line running of the vehicle, and this causes a lessening of the control of the vehicle by the driver.
To solve the above-mentioned problem, a sensing element 61 for detecting an angle of turn of the steering wheel 51, and a further sensing element 62 for detecting a stroke of the power cylinder 54, are both connected to an electronic controller 63 which constantly monitors whether or not a delay has occurred in the motions of the steering wheel and the power cylinder 54, and when an amount of such a delay exceeds a predetermined reference value, the controller 63 opens a solenoid-operated valve 64 to thereby permit the pressurized oil to flow from a high pressure side to a low pressure side (an oil tank side) of the power cylinder 54, and accordingly, the motion of the power cylinder 54 is delayed to compensate for the afore-mentioned angular error.
Nevertheless, when the electronic controller 63 is employed, detection signals generated by and transmitted from the sensing elements 61 and 62 must be accurate. Since the sensing element 62 for detecting the stroke of the power cylinder 54 is arranged adjacent to the tires of the rear axle of an industrial vehicle, dust and mud are apt to cover the element 62 during the running of the vehicle, and therefore, it is necessary to ensure that the operation of the sensing elements 61 and 62 is always reliable and accurate without suffering from an adverse affect of coverage by a foreign substance such as dust, dirt, and water. The two sensing elements 61 and 62 must be dust-proof and water-proof, and accordingly, these elements are expensive. Further, the electronic controller 63 using a micro-processor and associated electronic components must be protected against water and vibration, when mounted on the vehicle, and therefore, the electronic controller 63 is very costly. Moreover, the employment of the electronic controller 63 only for processing the detection signals of the two sensing elements 61 and 62 leads to an increase in the cost of manufacturing the industrial vehicle, i.e., a problem arises in that the conventional industrial vehicle becomes more expensive when the electronic controller for correcting the operation of the fully hydraulic power steering apparatus is accommodated therein.